For improving the selectivity of bandpass filters in SAW technology, it is known to interconnect DMS filters in parallel or in series with reactance elements in the form of resonators. For SAW filters, it is provided to realize these resonators on the same chip as SAW resonators in a single-gate or double-gate construction.
From U.S. Pat. No. 5,694,096 A, a two-sided, asymmetrically operated DMS filter is known, which is interconnected on the input side and output side in series with a single-gate resonator. From U.S. Pat. No. 5,770,985 A, it is known to interconnect a DMS filter with both a series and also with a parallel single-gate resonator. From WO2005/107065A it is known to interconnect an output-side, symmetrically operated DMS filter in series with an acoustic double-gate resonator.
For optimizing a filter arrangement that has a DMS filter and thus resonators interconnected in series or parallel, resonators with low capacitances can be necessary because they have particularly strongly impressed pole positions. For resonators with low capacitance, however, the problem appears that they are often realized only with low finger counts and/or small apertures. However, small apertures with a size of less than 20λ, expressed as a multiple of the average wavelength λ, increase transverse losses. On the other hand, resonators with less than 100 electrode fingers have increased reflector losses and thus generate increased total losses of the filter arrangement and also lead to undesired resonance peaks in the passband.